In the MRI apparatus, a gradient magnetic field is used to give position information to an echo signal from an object placed in the static magnetic field. Specifically, the coordinates of an echo signal arranged in measurement-space (k-space) are determined by the hysteresis of the gradient magnetic field in each direction that is applied when generating the echo signal. An image of the object is reconstructed by performing an inverse Fourier transform of the echo signal data that fills this k-space. The k-space coordinates are determined on the assumption that the gradient magnetic field pulse has a designed waveform, and the magnitude of the gradient magnetic field pulse is determined when calculating the pulse sequence that controls imaging.
However, there is error in the actual gradient magnetic field output. For this reason, since the echo signal is arranged at the coordinates shifted from the coordinates at which the echo signal is to be originally arranged in k-space, there is a problem in that the image quality is degraded. This problem is serious in the image according to the imaging sequence of the non-orthogonal system sampling method for sampling k-space radially or spirally, compared with the imaging sequence of the orthogonal system sampling method for sampling k-space in parallel to the axial direction.
Here, the error of the gradient magnetic field output means a difference between the amount of the gradient magnetic field pulse set at the time of sequence calculation and the amount of the actually output gradient magnetic field pulse (the amount of the gradient magnetic field given to the spins of nuclei within the tissue that form the object). As the cause of this error, various causes, such as static magnetic field non-uniformity or gradient magnetic field offset, eddy current, temporal shift of the gradient magnetic field output, and rising (or falling) time shift, are included.
Among these causes, the static magnetic field non-uniformity or the gradient magnetic field offset rarely changes depending on the sequence or the imaging parameter. Therefore, it can be calculated and corrected in advance, and shimming, offset adjustment, and the like are included as a pre-scan in many cases. However, since the eddy current, the temporal shift of the gradient magnetic field output, and the rising time shift change depending on the sequence or the imaging parameter in many cases, it is difficult to calculate and correct them in advance. In the present invention, these components that are difficult to calculate and correct in advance are collectively called a system response of the gradient magnetic field output.
As a technique for solving the error of the gradient magnetic field output due to the system response, there is a method of measuring a gradient magnetic field pulse used in imaging, calculating the waveform, and performing correction at the time of image reconstruction or a method of measuring the system response of the gradient magnetic field output in advance and correcting the signal using the value at the time of image reconstruction. Applying the latter method to the non-orthogonal system sampling method has been proposed in NPL 1. Specifically, the gradient magnetic field output (gradient magnetic field waveform) considering the system response of the gradient magnetic field output is calculated by calculating an equivalent circuit to approximate the system response circuit of the gradient magnetic field output and performing a convolution operation of a transfer function (more precisely, a function obtained by the inverse Laplace transform thereof), which is expressed by this equivalent circuit, to the gradient magnetic field output.